Early B cell development and late B cell maturation are regulated by signals emanating from the pre-B cell receptor (BCR) and BCR, respectively. During maturation, B cell tolerance to self-antigens is established through clonal deletion, receptor editing and anergy, which are controlled by signals emanating from the BCR. Distorted pre-BCR/BCR signaling often results in defective B cell development, breakdown of B cell tolerance and development of immunodeficiency and autoimmune diseases. Signals from the pre-BCR/BCR that regulate B cell development and tolerance are not fully understood. Phospholipase C3 (PLC3) is an important lipid enzyme involved in pre-BCR/BCR signaling. PLC3 has two isoforms, PLC31 and PLC32. PLC32-deficient mice are viable and have impaired early and late B development. Our recent data find that PLC32 plays an important role in activation of light chain loci, editing of self-reactive receptors and induction of B cell anergy. PLC31 deficiency results in early embryonic death at midgestation, precluding analysis of its role in B cell development. However, our studies of PLC32-deficient mice that are heterozygous for PLC31-deficiency (PLC31PLC32-/-) indicate that PLC31 also plays an important role in B development. We have recently generated mice in which the PLC31 gene can be conditionally inactivated. With the mice that have genetically modified PLC31 and PLC32 genes, we are well-positioned to further study the individual and combined roles of PLC31/PLC32 in B lymphopoiesis, including tolerance establishment, and the mechanism by which both PLC3s regulate the process. We hypothesize that both PLC31 and PLC32 play an important role in pre- BCR/BCR-mediated functions and in establishing B cell tolerance. To test our hypothesis, we propose three specific aims. We will 1) determine the role of PLC31 and combined roles of PLC31 and PLC32 in pre- BCR-mediated early B cell development, allelic exclusion of IgH chain, activation of the IgL chain loci, and formation of the B cell repertoire, 2) determine the individual and combined roles of PLC31 and PLC32 in BCR- mediated B cell maturation, receptor editing and induction of anergy in B cells, and 3) study the upstream and downstream pathways of PLC31 and PLC32 during pre-BCR/BCR signaling. The proposed research seeks to understand the roles for the two PLC3 isoforms in B lymphopoiesis, especially tolerance establishment, and the mechanism by which they relay the signals from the pre-BCR/BCR. The study may provide new clues to the molecular pathogenesis of autoimmune diseases and help identify targets for specific therapies. PUBLIC HEALTH RELEVANCE This proposal seeks to understand the roles of two important signaling molecules PLC31 and PLC32 in the development of B lymphocytes, including the removal of autoreactive B cells, and the molecular mechanism by which PLC3s regulate these processes. The study may provide new clues to the molecular pathogenesis of autoimmune and immunodeficiency diseases, and help identify novel targets for specific therapies.